Hydraulic servo mechanism for gas turbine fuel regulators



Nov. 17, 1953 N. E. STARKEY 2,659,349

HYDRAULIC SERVO MECHANISM FOR GAS TURBINE FUEL REGULATORS Nov. 17, 1953N. E. STARKEY 2,659,349 HYDRAULIC SERV@ MECHANISM FOR GAS TURBINE FUELREGULATORS Filed July 50, 1952 2 Sheets-Sheet 2 POSITION 5649 Inventor-zNeal E Starkey,

by unmix?.

' His Attorney.

Patented Nov. 17, v`1953 HYDRAULIC SERVO MEGHANISM FOR GAS TURBIN E FUELREGULATORS Neal E. Starkey, Schenectady, N. Y., assignor to GeneralElectric Company, a corporation of New York Application July so, 1952,serial No. 301,616 2 claims. 1(c1. iai- 41) This invention relates tohydraulic servo-devices, particularly t9 a combination of hydraulicservo-mechanisms for use in the fuel regulating system of `a thermal powerpl ant, such as a gas turbine.

While not necessarilylimited thereto, this invention is particularlykadapted for use in a gas turbine fuel system of the general 4type shownin the copending application of Bruce O. Buckland, Serial No. 183,332,1ed September 6, 1 950 and assigned to the same assignee as the presentapplication. Generally, such a fuel system ccmprises a variabledispiacemerit pump connected to Supply a liquid .fuel'sugh as diesel 0rBunkerfC oil to a plurality o fnoazles in the gas turbine combustionsystem. The displacement of the fuel pump is automatically varied lby ahydraulic servo-device actuated by a variable contro1 oil pressuresignal, vwhich is supplied `bya regulator containing a complexcombination of various condition-responsive servo-devices cooperating'to produce the variable control oil pressure. Hereinafter, thisvariable pressure signal which acts on the fuel pump servo todeterminethe rateof fuel supply will be referred to as the VCO pressure.

Another gas turbinefuel system of atype which may protably incorporatethe presentinvention is illustrated in United States patent to N. E.Starkey et al., 2,558,592,1issued June-26, 1951,.and assigned to thesame assignee as the presentrapplication.

The general arrangementlof the specific powerplant for which the presentinvention-Was developed, and other details of the regulating systemtherefor are disclosed in the United States patent of George R. Fusnerand Chapman J. Walker, No. 2,617,253, -issued November 1'1, 1952, andassigned to the same assignee as the present application. Thispowerplant is characterized by a two-stage gas turbine withmechanicallyindependent rotors, one of which drives a low pressurecompressor inseries with a high pressure compressor driven by' the otherturbine rotor. The fuel pump is alsovdriven from thehigh pressurecompressor rotor. 'I his means that, for a given setting of the fuelpump regulator, the rate of fuel supply will be a function of vhighpressure compressor shaft speed.V As will beappreciated by those skilledin the art, the volume rate of air flow through the -high pressurecompressor to the combustion system is al function'` of the rotationalspeed ofthe highpressure compressor. -But, since the loW pressurecompressor is driven ata variable speed by its independent turbinerotor, the weight `ing a hydraulic piston rcondition by thek VCO,calibrated s pring,.=.vith aY pilot valve for establsure responsivebellows rate of air flow through the system is also a function of thedensity or'pressure at theinlet tothe high pressure compressor.Thereforein order to make the Afuel regulating system accuratelyresponsive to the Weight vliow of air, it isnecessary that the fuelregulating system incorporate means for sensing the pressureat the inletto the high pressure,compressorl Accordingly, ytheobject of the vpresent invention iS t0 provide an imnroredhydlaulc SCWO-mecha# nism fora gas turbine fuel regulatingsystemof the type described including meansA'for modify- .ine lthe VCO ,fuel rate pressure .Signal in attardkancewith the inlet pressure of the high pressure compressor.

A further object is to provide an improved hydraulic regulatingcomponent of the ytype described, kin which the VCO pressure -isbalanced directly againsta calibrated fmainspring'f.y and ,the.modifying are@ ofthe high Mesure 0,9m,- pressor inlet pressureisintroduced ata point between the VCO .piston and .the Pilot velrerhicll,determines v,th-'e VCO pressure, in order to `improve the SensitivityQ'f' ihe'ofhiinisrrby 'fedro- .ing friction effects in thecompressorpressure responsive modifying mechanism.

Other objects and advantages will become apparent from ythefollovvingdescription takenin connection Withthe accompanyingdraivin'gsJ in which Fig. 1 is a diagrammatic representationof a gasturbine powerplant vvith a fuel system, hav ing, `hydraulicservomechanism in accordance with theinvention, the servo-mechanismbeing shown in section in more detail, and Fig'. 2 a graphicalrepresentation of the performance of the regulatin g mechanism to showtheeiectof the invention.

Generally, the invention is practiced by provid' maintained inequilibrium pressure acting against a lishing the VCO pressure, and acompressor pres- ,connected to produce a .modifying effect on thelinkagewhichconnects .theVCO piston withthe pilot valve.

Referring now more; particularly toFig. l, the

.inventionis show-nas applied to agasturbine powerplant .comprising a10W pressure compressor Liconnectedin series kwith a high pressurecompressor 2. a combustionsystem, and v,atwostage turbine vIl. Theconduit Econnecting the compressors contains an intercooleri, andtheconduit connecting the high pressure compressor with the combustionsytinbntain's'a regenerator 6a in which Waste heat from the exhaust gasis transferred to the high pressure air on its way to the combustors. Itis to be particularly observed that the two-stage turbine 4 comprises afirst stage rotor a directly coupled to the high pressure compressorrotor, and a mechanicahy independent second-stage rotor 4b directlycoupled to the low pressure compressor rotor.

rlhe variable displacement fuel pump is shown at 1 as connected to anaccessory drive pad 8 and driven at a speed proportional to the speed ofthe high pressure compressor by suitable gearing (not shown). Alsodriven from the accessory pad 8 is a second positive displacement pump9, the function of which is to provide hydraulic operating liquid forthe fuel regulating system. It will be understood that the pump 9furnishes control oil at a constant pressure, as determined by suitablepressure regulating valve means (not shown), and this constant pressurecontrol oil will be referred to hereinafter as constant control oil(CCO) The variable pressure signal (VCO) for altering the displacementof the fuel pump 1 is communicated to a hydraulic servo-device indicateddiagrammatically at 1a in Fig. 1. 'Ihe exact mechanism by which thisservo determines the stroke of the fuel pump need not be described here,being disclosed more particularly in the above-mentioned application ofB. O.

Buckland, Serial No. 183,332. In order to understand the presentinvention, it is necessary only to note that the VCO pressure suppliedto the servo-device 1a varies the stroke of the fuel pump 1 so that thefuel supply rate to the combustion system is a joint function of the VCOsignal pressure and the high pressure compressor shaft speed. In Fig. 1,the fuel pump is shown connected by a conduit I with a spray nozzle IIin combustor 3a. It will of course be understood that similar fuelsupply conduit means are provided for the other combustors. The detailsof the combustion chambers, fuel nozzles, and the arrangement of pipingthrough which the fuel pump supplies oil to the combustors are notmaterial to this invention.

Referring now to the hydraulic servo-mechanism to which the presentinvention specifically relates, it will be seen in Fig. 1 that thehousing I2 defines a bottom chamber I3 closed by a cover plate I3a, atop chamber I4 closed by a cover plate I4a, a left side chamber I5 witha cover plate I5a, and a right side opening I6 closed by cover plate Ia.The constant pressure operating liquid is communicated by the CCO supplyconduit 9a to an inlet port I1 in the left-hand side of the housing, anda second inlet port I8 in the bottom of the housing. The VCO signalpressure, which is the useful output" of the device, is communicated tothe fuel pump servo 1a by the VCO conduit 1b, which communicates asshown in dotted lines with a port I9 located approximately at thegeometric center of the housing I2.

The hydraulic servo-device comprises the VCO piston assembly indicatedat 20, the VCO pilot 2|, and the compressor pressure responsive servoindicated generally at 22. Generally stated, the function of thesecomponents is that the pilot 2| controls the supply of CCO operatingliquid to the VCO piston 20, and the compressor pressure compensator 22alters the characteristics of the linkage which connects the VCO pistonwith the VCO pilot.

Referring now more particularly to the construction of the VCO pistonassembly 2D, it will be seen that the movable piston member 23 is'actually a plunger member denning a cylindrical recess 24 in which isshdably disposed a piston member 25. Piston 25 does not movelongitudinally relative to the housing I2, but is rotataoly supported ina bushing member 26. The projecting end portion 25a carries a gear 21,the function of which will be seen hereinafter. Piston 25 may beprovided with a plurality of circumferential grooves 28 for improvinghydraulic balance, in a manner which will be understood by those skilledin the art. The piston 25 and the moving cylinder member 23 cooperate todefine a pressure chamber 2s to which the VCO signal pressure iscommunicated by a transverse drilled hole shown in dotted lines at 3Ucommunicating with a longitudinal drilled hole 3 I.

The VCO pressure existing in chamber 29 produces an upward force on theVCO piston 23 which is balanced against a carefully selected coil spring32. This spring surrounds piston 23 with the lower end thereof engaginga radial end flange 23a, the upper end of the spi-mg seating in acylmdrlcal recess 33a of a threaded bushing member 33. It will beapparent in rig. 1 that the moving VCO piston 23 passes freely through acylindrical opening 33b in bushing 33, with generous radial clearancetherebetween. Thus, it will be seen that piston 23 is positioned freelyby the VCO pressure in chamber 29 balanced against the calibrated spring32. The force exerted by spring 32 may of course be adjusted somewhat byvarying the depth to whichthe threaded bushing 33 is screwed down intothe housing portion receiving it. The lowermost position of piston 23 isdetermined by engagement of the lower end flange 23a, with a stop member34, which may be an anti-friction bearing slipped over the reduceddiameter piston portion 25h before piston 25 is inserted in the bushing25 and the gear 21 assembled. It will be apparent in Fig. 1 that gear 21is keyed to the shaft end portion 25a, and may be retained by anysuitable means such as the snap- It is also to be noted that the VCOliquid is supplied to an annular groove 26a in the outer circumferenceof bushing 26 by way of a passage I9a communicating with the VCO portI9. From the annular channel 26a, liquid is communicated by a pluralityof circumferentially -clearances between bushing 26 and the housing isprevented by a pair of O-ring seals 35, 38, the arrangement of whichwill be obvious from the drawing.

The condition shown in Fig. 1 represents the minimum VCO pressure, withpiston 23 at the bottom of its range of movement against the stop member34.

The VCO pilot assembly 2l comprises a longitudinally movable pilotspindle 31 having a pair of axially spaced lands 31a, 31b connected by areduced diameter portion 31c and slidably disposed in a bushing 38,having an axial bore 38a. Bushing 38 is rotatably disposed in a secondbushing member 39, which may be retained in place in the housing by anupper end flange 33a and a snap-ring 39h engaging the lower end thereof.Keyed. to the upper ejnd of the rotatable bushing 38 is a gear 40; and asecond gear 4| is keyed to the lower end of bushing 38 s0 asv to engagevwith gear 21 on the rotatable piston member 25.l Gear 40 may beconveniently retained by snap-ring 40a, and gear 4| may be secured bysnap-ring 4|a. i

The purpose of these gears is to effect continuous rotation of the pilotbushing 38 and the piston member 25 in order to reduce friction effectsbetween piston 25 and the VCO piston 23, and between the pilot 31 andits bushing 33, respectively. The idea of thus rotating one of a pair ofcooperating sliding parts to reduce friction effects therebetween is nota part of the present. invention. Such rotational movement of bushing38,` and piston 25 is produced by a worm gear 42 carried on a shaftsuitably journaled in the housing I2,y the mechanical details of whichare not important to the present invention.

Operating liquid is supplied from the CCO inlet port |8 by way of apassage lila to an annular groove 43, communicating by Way of aplurality of circumferentially spaced ports 44 with cooperating ports 45in bushing 38. The annular space 31d surrounding the pilot spindleportion 31o communicates by way of ports 46 with an internal annulargroove 41 and a plurality of spaced` ports 48 in bushing 39. Port 48communicates by way of conduit |9b in the housing with VCO port |9.

Bushing 38 also denes a plurality of spaced ports 4,8 cooperating withports 58 and an annular drain groove For eiectingr free discharge ofspent operating liquid from groove 5|, the housing is provided with anaxially extending slot shown at 52. This permits free egress ofoperating liquid past the end flange 39a and the generously proportionedrecess in which gear 43 is disposed.

The compressor pressure compensator 22 comprises a pressure responsivebellows assembly indicated generally at 22a and a hydraulicforceamplifying servo indicated generally at 22h.

The bellows assembly 22a comprises a cylindrical housing member 53threadedly received in cover plate |3a and secured in adjusted positionby a lock-nut 54. The axial bore through the tube member 53 contains anend seal member 55 retained by an internal snap-ring 56, with iluidleakage between the tube 53 and seal ring 55 prevented by an O-ringrseal 51. Disposed in the tubular housing 53 is a flexible bellows 58having its lower end sealed to the ring member 55, high pressurecompressor inlet pressure being communicated by conduit 5b to theinterior of bellows 58. The upper end of the bellows is sealed to atting 59 which serves as a seat for a biasing coil spring 60, the upperend ofwhich abuts a washer 6| secured by a snap-ring 62. The bellows endtting 59 also carries a transverse pivot 63 by which the motion of thebellows is communicated to the pilot rod member 64.

The hydraulic amplier assembly 22h comprises the pilot rod 64, havingaxially spaced landsv 64a, 64b separated by a reduced diameter portion64C. Pilot 64 is slidably disposed in the axial bore 65a of a pistonmember` 65 which is in turn slidably disposed in a cylinder member 66.Cylinder 6,6 may be conveniently retained ina lrecess in the housing byan internal snap.-

'ring shownat 61` The upper portion of cylinder 65 denes a chamber 66a,the upper end of which pressure supplying Whatever is closed by a vsealring 68.- Rns member 68 may be retained by ari internal 'Snap-rios 6,8and nuid maltese between ring 68 and the .Outer surface of piston member65 is prevented by Van Owing seal 68a. It will be seen that the pistonportion 65h of the member 65 cooperates with recess 66a to denne anupper pressure chamber 10 and a lower pressure chamber 1|.

The hydraulic passages in the amplifier 22h are as follows. Theconstantv pressure CCOr oil from inlet port |1 is supplied to an annulargroove 12 in the outer circumference of cylinder member 66, whence itpasses by way of several drilled holes 13 to an internal annular groove14, which communicates with several circumferentially spaced holes 15 inpiston member 65. By way of these passages, CCO oil is suppliedcontinuously to the annular space 16 der lned between the spindleportion 64e and the bore 65a of the piston 65. Downward movement ofpilot spindle 64 causes the lower land 64a yto uncover another set ofports 11 so that oil is admitted by way of a communicating axial pas.-sage 18 to the upper chamber 10 deiined above piston 65h. Such downwardmovement of pilot spindle 64 also causes the upper land 64b to uncover aset of drain ports 13, so that liquid trapped below the piston 65hleaves the chamber 1| and is discharged through the top portion of bore65a and through one or more transverse drilled holes 80.

Conversely, upward movement of pilot spindle 64 causes operating liquidfrom thev annular space 16 to be supplied by way of the passages 18 tothe lower chamber 1|, forcing piston 65h upwardly, while operatingliquid trapped in chamber 10 leaves by way of theaxial passage 18,beingdis.- charged downwardly past the land 64a and out through thegenerous clearance space dened between spindle 64 and the adjacent wallof bore 65a.

Those skilled in the hydraulic servo-mechanism art will appreciate thatthis arrangement is merely a hydraulic power amplier for producing adisplacement of piston 65 proportional to the input movement of thepilot spindle 64 effected by the iiexible bellows 58. This permits useof a comparatively small and light bellows 58, since the bellows needexert a force only suilcient to position the pilot spindle 64, hydraulicpower is required to effect the compensating adjustment described below.

The mechanical linkage by which the VCO piston 20 is interconnected withthe VCO pilot 2i and the compressor pressure compensator 22 produces itsmodifying effect is as follows.

The principal member of the interconnecting linkage is a floating lever8| having a middle portion connected by a pivot 82 to the upper end ofVCO pilot spindle 31. The right-hand end of lever 8l carries a camfollower roller 8-3 engaging a contoured cam 84 carried on a transverseshaft 35 journaled in the housing and arranged to be positionedbykexternal means, the precise nature of which need not be noted here.For purpose of illustration, the cam positioning means is represented bythe manual handle 85a.. It is to be noted that the handle 85a is in theon position, with cam 84 rotated clockwise as far as it will go, so asto position cam follower 83 to the uppermost point in its range ofmovement.

.In simpler, uncompensated, hydraulic .servomeehanisms of this generaltype, the left-hand end of lever 8| bearsdirectly against the roundedppere'nd 23h of the VCO piston 23. In accordance with the presentinvention, however, there is interposed between piston end portion 23hand the lever end portion 8|a a transversely adjustable roller 88. Thisadjustable abutment roller is journaled at the right-hand end of a lever81 which is, at its left-hand end, pivoted at 88 to a shiftable blockmember 89 slidably disposed between upper and lower guide members 80,8|. The central portion of block 89 is provided with a transverse recess92 across which extends a pivot member 93 engaging the forked end of alever 84, which is in turn carried on a pivot 95 secured in the housing.As seen in Fig. 1, lever 94 pivots from the position shown in full linesto that indicated in dotted lines at 94a. The lower guide member 9| isprovided with an opening BIa which is of course of sufiicient size thatmotion of the lever 94 will not be obstructed.

The lever 94 is actually a sort of bell-crank havving a forked lower endportion 94h carrying a transverse pivot 86. Pivot 9G passes through atransversely elongated slot 91 formed in the extreme upper end portionof piston member 85.

It will be apparent from the above description of the mechanism thatvertical movement of piston 85 will cause bell-crank lever 94 to rotateabout pivot 95 so as to effect transverse sliding movement of block 89and adjust the abutment roller 89 transversely relative to VCO piston23. It is also to be noted that the adjustable bloei: 89 is shown in itsextreme right-hand position. The extreme left-hand position correspondsto the dotted lever position 94a, in which roller 8S is in the positionindicated by dotted lines at 88a as being directly over the end portion23h of the VCO piston. It will be understood that, when the abutmentroller is in this dotted position 88a, the compensating mechanism has noeffect on the lever 8|, and the VCO piston 23 acts on lever 8| preciselyas if the compensating roller 8B were not present. On the other hand,when roller 88 is displaced to the right of the dotted line position35a, a special compensating eiect is introduced into the linkage, thedegree of this compensation being of course proportional to that calledfor by the compressor pressure responsive bellows 58.

In order to maintain lever 8| properly seated against the roller 88 andthe cam 84, a comparatively light coil spring 98 is interposed between asocket formed in the top cover plate |4a and the upper end fitting 31fof pilot spindle 31. rl'he downward force of spring 98 is justsufficient to maintain good contact between lever 8| and the roller 88and cam 84 respectively, this spring force being so small as to have noeffect on the positioning of VCO piston 23, etc.

In normal operation, spent operating liquid draining from the hydraulicamplifier 22h and from the VCO pilot 2| completely fills the chambersI3, I4, I5, I8 and returns to the operating liquid supply system throughdrain conduit |3b. Thus all moving parts operate submerged inlubricating oil.

The integrated operation of this servo-mechanism may be outlined asfollows.

Assume first that the powei'plant is shut down, which means, of course,that there will be no compressor discharge pressure in the bellows 58,and no hydraulic motive uid supplied to the CCO supply conduit 9a.Therefore, the flexible bellows 58 will be collapsed by the compressionspring 68, so that pilot 84 is in its lowermost position with land 84abelow the port 11 and land 84h below the drain port 18. Without CCOpressure, the VCO piston 23 will also occupy its lowermost position, asshown in Figure l, by reason of the downward biasing force of themain-spring 32. Finally, the fuel rate selecting cam 34 will be in itsmaximum clockwise position, with cam follower 83 in maximum elevatedposition and lever 8| occupying the position shown in the drawing.

If now the powerplant is started and the fuel rate selecting cam 84 iscaused to rotate counterclockwise, the cam follower roller 83 willdescend, lever 8| will pivot clockwise about the abutment roller 88,with the result that pivot 82 causes VCO pilot spindle 31 to descend,admitting oil to the VCO chamber I9, and by way of passages 30, 3| tothe pressure chamber 29 in VCO piston 23. Increasing pressure in chamber29 causes piston 23 to rise against the bias of spring 32, thus causinglever 81 to pivot counterclockwise about pivot 88, so that roller 88causes the left-hand end of lever 8| to rise. This motion restores pilot31 to the aligned condition, in which the lower land 31h blocks off theCCO inlet port 45. It follows that, as cam 84 continues to rotatecounterclockwise, the VCO pilot 31 will be positioned to establish anincreasing VCO pressure in the chamber 29, piston 23 being progressivelypositioned upwardly as the VCO pressure increases. Thus it will be seenthat there will be a preselected VCO pressure corresponding to apreselected position of piston 23 ior cach angular position of the inputcam 84.

Meanwhile, the CCO pressure supplied to the compensator servo 2.2b hasentered the upper chamber 'i0 and forced piston 85 downwardly to itslowermost position, corresponding to the completely collapsed conditionof pressure responsive bellows 58. This causes the abutment roller 88 tobe positioned to its extreme righthand condition, as shown in full linesat 88, so that the maximum degree of compensating effect is introduced.Then, as the high pressure compressor inlet pressure rises into thenormal operating range, increasing pressure in bellows 58 causes thehydraulic amplifier 22h to effect progressive positioning of lever 84counterclockwise about pivot 95 so that the abutment roller 88 ispositioned progressively to the leit, meaning that the degree of thecompensating elect introduced progressively changes.

Once the machine is operating in its normal range, it will be seen thatany change in the high pressure compressor inlet pressure, due tochanges in speed of the low pressure compressor I, will be accompaniedby a change in the position of the abutment roller 85 so as toappropriately modify the follow-up action of the VCO pislpn 23 on lever8|.

The net eiiect of the above-described method of operation may be seengraphically in Fig. 2. Here the abscissa represents rotationaldisplacement of the input cam 84 from the minimum or off position shownin Fig. l to its maximum counterclockwise position corresponding to themaximum position of handle 85a. The ordinate represents the VCO signalpressure communicated from port I9 by way of conduit 1b to the hydraulicservo 1a of the fuel pump. As noted on the curves in Fig. 2, the loweicurve represents the performance of the apparatus with the abutmentroller 88 in the full line position 86 shown in Fig. 2, while the uppercurve represents the performance with the abutment roller in the dottedline position 88a, directly over the VCO piston 23. Actually, thesecurves are not curved at all but are straight lines, the contour of theinput cam 84 being so chosen that straight-line performance curves willresult. This maires it easier to design the fuel system and the fuelpump control servo 1a.

Thus, it will be seen that shifting the abutment roller 86 acomparatively small distance has a very substantial effect on theoperating characteristics of the combination.

The invention makes it possible to so design the regulating mechanismthat a preselected fuel-air ratio is obtained for any given position ofthe input cam 84, regardless of variations in the operating speed of thelow pressure compressor I. This means that, for each position of inputcam 84, there will be a preselected rate of fuel supply which isproperly proportioned to the weight fiow of air to the combustionsystem, irrespective of any variations which may occur in the speed ofthe independently driven low pressure compressor l.

It will be obvious to those acquainted with the art that manymodilications and substitutions of mechanical equivalents may be made.For instance, the hydraulic amplifier 22D might be replaced by anysuitable means for positioning the compensating lever 94 as a functionof high pressure compressor inlet pressure. Also, it may be noted thatthe compressor pressure signal may be derived from the conduit 5anywhere between the discharge end of the low pressure compressor andthe inlet casing of the high pressure compressor. It will be obviousthat the mechanical details of the VCO piston, and of the VCO pilot, andthe linkage interconnesting them may also take many other forms. It isof course intended to cover by the appended claims all suchmodifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In hydraulic servo-mechanism, the combination of a housing, a firstreciprocable piston member having a cylindrical chamber in one endthereof and containing a second non-reciprocable piston member deiiningWith the slidable piston a pressure chamber, the second piston having a.passageway for communicating liquid to said chamber, a main coil springsurrounding the first piston and having an end portion engaging thepiston to exert a force thereon in opposition to the pressure'in saidchamber, a source of operating liquid at substantially constantpressure, pilot valve means for supplying liquid from said source tosaid passageway and including a pilot spindle disposed for longitudinalsliding movement in a direction substantially parallel to the axis ofsaid pistons, linkage means connecting the first piston to position saidpilot spindle including a iirst lever member disposed generally normalto the exis of the piston and pilot and having an intermediate portionconnected to position the pilot spindle, an input member engaging theend of the lever remote from the first piston, and variable abutmentmeans interposed between the other end of the lever and the firstpiston, said abutment means comprising a slider member adapted to bepositioned linearly in a direction generally parallel to the lever, anda second lever member pivoted at one'end to said slider and having amid-portion engaged by the end of the first piston, the other end ofsaid second lever having a member engaging the adjacent end portion ofthe first lever, and means for positioning said slider towards and awayfrom the first piston to vary the position at which said second leverend portion engages the first lever relative to the axis of the piston.

2. Hydraulic servo-mechanism in accordance with claim l in which themeans for positioning the slider comprises a pressure signal responsivebellows connected to a pilot spindle, the pilot spindle being slidablydisposed in a double-acting hydraulic servo-piston member With portscontrolled by the pilot spindle and passages for supplying operatingliquid to either side of the piston alternately, and lever meansconnecting said servo-piston with the slider whereby the latter ispositioned in accordance With said pressure signal.

NEAL E. STARKEY.

References Cited in the lile of this patent UNITED STATES PATENTS

